374. Guess Number Higher or Lower 

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public class Solution extends GuessGame {
    public int guessNumber(int n) {
        int left = 1, right = n;
        while (left < right) {
            int mid = (left + right) >>> 1;
            int guess = guess(mid);
            if (guess < 0) {
                right = mid - 1;
            } else if (guess > 0) {
                left = mid + 1;
            } else {
                return mid;
            }
        }

        // now left and right are equal
        return left;
    }
}

References

374. Guess Number Higher or Lower